A variety of printing systems exist today that employ liquid inks requiring drying after they are applied to a print media or substrate.
Aqueous inks used in ink jet printing as well as other printing processes have water as a major component. Although water has the advantage of being non-toxic, environmentally friendly, and an excellent solvent for dyes, it has the disadvantage of interacting with cellulose fibers in wood-based papers to cause two major distortions known as paper cockle and paper curl. Paper cockle is a distortion in which bumps, indentations, and other irregularities produced on the printed paper give the paper a wrinkled appearance. Curl is a phenomenon in which the edges of the paper migrate towards the center of the paper. It is desirable to remove the moisture quickly from the deposited ink and substrate so as to fix the ink to the substrate with a minimal amount of cockle and curl.
Printing processes have been developed that attempt to speed the drying of ink following application to the print media. One approach to drying is to use highly volatile solvents in the ink jet inks (alcohols, ethers, etc.). However, there are issues of toxicity and flashpoint related to the use of these components. In addition, many ink formulations using this approach generally give poorer text quality on plain paper media due to poor edge acuity.
Another approach to drying involves the addition of heat to effect removal of the moisture content of the ink. Existing printing processes employing conductive heaters are not very efficient because extra power must be supplied to compensate for thermal losses to the environment and print media. Also, if the conductive heater is located in the printing zone, it typically impacts reliability in a negative fashion. In printing processes employing convective heating, hot air is blown on the print media to dry the ink. However, convective heating is even less efficient than conductive heating and if the velocity of the hot air is too high, it can misdirect ink droplets and cause image quality defects. Infrared radiation is another option in delivering thermal energy to the print media, but presents a fire hazard if for any reason the print media (e.g., paper) being printed upon stops moving through the printer.
In addition to the foregoing problems, thermal heat impacts the print media. Problematic thermal effects include paper dimensional changes such as anisotropic shrinking, dry cockle and curl. Pre-heating is also typically required to prevent the paper from shrinking in the print zone while being heated. Therefore, research has focused on inks that dry more quickly while minimizing dimensional changes in the print media.
Another approach to drying inks in printing processes is to use microwave energy to heat the inks. Because current inks do not absorb microwave energy well, microwave applicators are required to be very large to allow for increased dwell time in the microwave cavity. In traditional printing processes employing heating, the maximum temperature that the print media can withstand (˜80° C. for special paper media) dictates the maximum heater temperature.
A few inks specifically intended for ink jet printing processes employing microwave radiation are known.
Thus, a need remains for improved ink compositions and printing processes employing microwave drying that result in quick removal of the moisture content from the print media to which an ink has been applied. Improvements in drying time and reductions in print media dimensional changes and microwave applicator size and power requirements would be welcome benefits.